Possible reasons I'm loosing spark?

Replaced ignition module, cap & rotor, spark plugs, alternator, water temp sensor, battery, voltage regulator and car still looses spark usually after heating up. I ran the spark test light on it & thats how I found out it was loosing spark. Leaves me stranded sometimes. So far I put about 500-600$ into this. Please help....?

Oh ok I got ya... I replaced ignition module, voltage regulator was busted and alternator was pumping 18V, fried 2 battery's, changed voltage regulator, still have the problem, runs like a champ up until car gets hot, then rpm's bounce around for a minute until it stalls. Will start right back up, run for a min & then stall again, I cleaned the wire boots to the coil but no solution yet!!!